The present invention relates to an objective lens for an optical pick-up employed in an optical disc drive using at least two kinds of optical discs having different data densities. Conventionally, optical discs having different data recording densities have been known. For example, a DVD (digital versatile disc) has a greater data density than a CD (compact disc).
It is generally known that the data density of an optical disc and a beam spot formed on the optical disc are closely related. The beam spot is required to have a predetermined size in relation to a width of a track on the data recording surface of the optical disc. The track width is smaller for the high data density optical disc, while it is greater for the less data density optical disc. If the spot size is too large with respect to the track width, information recorded on adjoining track(s) may be read out as a noise. In a disc system which utilizes diffraction of light for reproducing data signals as in the CD system, if the spot size is too small, a sufficient diffraction effect may not be obtained, and the recoded data may not be read out correctly.
The size of the beam spot is smaller if the wavelength is shorter and an NA (numerical aperture) of an optical system is greater. Therefore, for the DVD system, an optical system is required to have a large NA and to use a light beam having a relatively short wavelength. For the CD system, an optical system is required to have a relatively small NA and to use a light beam having a relatively long wavelength. In particular, if a CD-R (CD recordable) is to be used, the wavelength of the light beam should be approximately 780 nm or longer in view of a reflection characteristic of the CD-R.
Therefore, in order to satisfy the requirement regarding the wavelength, an optical disc drive capable of using the DVD and CD-R is required to use two light sources capable of emitting a shorter wavelength beam (e.g., approximately 650 nm) for the DVD, and a longer wavelength beam (e.g., approximately 780 nm) for the CD (or CD-R).
In order to deal with the requirement of the NA, in a conventional optical system for a CD-DVD compatible optical pick-up, an aperture control mechanism is employed to achieve the required NA. For controlling the NA, a variable aperture or wavelength selective filter may be employed so that the beam size is changed depending on whether the CD is used or the DVD is used. However, in such a structure, i.e., the aperture mechanism or the filter is employed, due to increase of the number of parts, the weight and size of the optical system increases.
There has been conventionally suggested an objective lens provided with a diffraction lens structure for the optical pick-up applicable to the DVD-CD compatible optical disc drive. In such an objective lens, the wavefront of the light beam passed through the diffraction lens structure has a wavelength dependency such that appropriate wavefront is formed according to the two types of optical discs since the wavelengths of beams for the optical discs are different. In this conventional art, the diffraction lens structure generally has a plurality of annular zones, the boundaries of which are obtained based on an OPD function such that, at the boundaries, the amount of the optical path length added by the diffraction lens structure equals to an integer multiple m of the blaze wavelength. Such an objective lens, of which the boundaries of the annular zones are determined as described above, uses an m-th order diffraction light.
In the conventional objective lens formed with the diffraction lens structure, if the boundaries of the annular zones are designed as described above, the diffraction lens structure formed in the exclusive high NA area may not diffuse the beam incident thereon sufficiently when a beam for the low data density disc is used. Therefore, in some cases, when the low data density disc is used, noises may be generated.
The present invention is advantageous in that it provides an objective lens for an optical pick-up which does not require the aperture mechanism or the filter which has been employed in the conventional optical system.
In order to provide the above advantages, according to an aspect of the invention, there is provided an objective lens for an optical pick-up, which consists of a refractive lens element provided with a diffraction lens structure on at least one surface of the refractive lens element. The diffraction lens structure has a plurality of annular zones having steps therebetween. The objective lens is capable of converging at least two beams having different wavelengths on at least two types of optical discs (e.g., DVD and CD) having different data recording densities, respectively. The objective lens is partitioned into a common area through which a beam with a low NA corresponding to a low data density optical disc passes, and an exclusive high NA area which is designed to converge a beam with a high NA corresponding to a high data density optical disc. A base curve and an optical path difference function for the exclusive high NA area are determined so that a beam having a wavelength required to the high data density optical disc is sufficiently converged on the high data density optical disc. The boundaries of at least a part of the annular zones formed on the exclusive high NA area are designed independently from boundaries obtained from the optical path difference function while substantially keeping the base curve so that the beam with the high NA is converged substantially on a certain point and the beam with the low NA is diffused.
With the above configuration, the exclusive high NA area substantially functions to restrict the diameter size of the beam for the disc having the low data recording density.
In order to achieve the function to restrict the diameter size of the beam, the diffraction order the exclusive high NA area utilizes should not be determined to be a fixed order, such as second or third. Rather, the balance of light passing through the entire exclusive high NA area. It should be noted that a light source for an optical disc drive can be considered to emit a monochromatic beam. Therefore, the exclusive high NA area may utilize components of a plurality of diffraction orders.
For the beam corresponding to the high data density optical disc, the annular zones formed within the exclusive high NA area should be configured such that light passed through the exclusive high NA area is converged substantially at one point. It should be noted that designing such a configuration has certain degree of freedom. Accordingly, with maintaining the above condition, the width of each annular zone may be determined individually, a configuration which sufficiently diffuse the beam corresponding to the low data density optical disc can be obtained. Generally, the deviation of the wavelength of a laser beam ranges approximately 3% with respect to the design wavelength. Therefore, even though the locations of the steps obtained from the OPD function are shifted, if the change is within several times the wavelength with respect to the base curve, a desired wavelength characteristics can be achieved.
Generally, the beam with the low NA has a first wavelength, and the beam with the high NA has a second wavelength, the first wavelength being larger than the second wavelength. Typically, the two types of the optical discs are CD (or CD-R) and DVD. The beam for the CD has a wavelength of approximately 780 nm, while the beam for the DVD has a wavelength of approximately 650 nm. That is, the first wavelength (i.e., 650 nm) is less than 90% of the second wavelength (i.e., 780 nm).
According to another aspect of the invention, there is provided an objective lens for an optical pick-up, the objective lens converging at least two beams having different wavelengths to form beam spots having different sizes on optical discs having different data recording densities, respectively. Such an objective lens has a refraction lens having a positive power, and a diffraction lens structure having a plurality of annular zones formed on at least one surface of the refraction lens. The objective lens is partitioned into a common area through which a beam with a low NA corresponding to a low data density optical disc passes and an exclusive high NA area through which only a beam with a high NA corresponding to a high data density optical disc passes.
The diffraction lens structure formed in the common area exhibits spherical aberration which has a wavelength dependency such that variation of aberration due to difference of cover layers of the two types of optical discs is canceled by difference of wavelengths of the beams.
The diffraction lens structure formed in the exclusive high NA area has a plurality of annular zones, and exhibits no aberrations with respect to a beam converging point for the optical disc having the high data recording density for the beam used for the optical disc having the high data recording density.
The plurality of annular zones includes at least one wide-width zone which is configured such that, with respect to a beam converging point for the disc having the low recording density, a difference of optical path lengths at inner side end and outer side end of the at least one wide-width zone is equal to or more than 0.6 times the wavelength of the beam used for the optical disc having the low data recording density.
With this configuration, since the diffraction lens structure formed in the exclusive high NA area provides no aberration for the disc having the higher data recording density, it provides wavefront aberration for the disc having the lower data recording density. Thus, the exclusive high NA area of the objective lens diffuses the light passed therethrough when the disc having the lower data recording density is used. Preferably, in this case, the phase of the light having the longer wavelength and passed through the exclusive high NA area is not concentrated in a certain direction but evenly distributed (i.e., 360xc2x0). If the phase is evenly distributed, the light passed through the exclusive high NA area can be well diffused, and the diameter of the beam can be restricted substantially.
Optionally, the diffraction lens structure formed in the exclusive high NA area may be configured such that a step is formed at each boundary of the at least one of the wide-width zones and an other portion, the step providing an optical difference of m times the wavelength of the beam for the disc having the high data recording density, m being an integer not less than two.
Further optionally, at least one wide-width zone may consist of m of the plurality of wide-width annular zones. The m of the plurality of wide-width annular zones being arranged stepwise such that each of the m of the plurality of wide-width annular zones provides an optical path length difference of one wavelength of the beam for the disc having the high data recording density.
In this case, at least one narrow-width annular zone which may provide an optical path difference of less than 0.6 times the wavelength of the beam for the disc having the high data recording density is arranged between the m of the plurality of wide-width annular zones.
The objective lens may be configured to satisfy the conditions:
10xcexh/(n0xe2x88x92n1) less than Ww; and
xcexh/(n0xe2x88x92n1) less than Wn less than 10xcexh/(n0xe2x88x92n1),
wherein xcexh represents a wavelength of the beam for the disc having the low data recording density, n0 represents a refractive index of the air, n1 represents a refractive index of the refraction lens element, Ww represents a width of each of the m of the plurality of wide-width annular zones, and Wn represents a width of each of the at least one narrow-width annular zone providing an optical path difference of less than 0.6 times the wavelength of the beam for the disc having the high data recording density.
Still optionally, at least one of the plurality of annular zones may include a plurality of wide-width annular zones, each of which is configured such that, with respect to a beam converging point for the disc having the low recording density, a difference of optical path lengths at inner side end and outer side end of the at least one of the plurality of wide-width annular zones being equal to or more than 0.75 times the wavelength of the beam used for the optical disc having the low data recording density.
Further optionally, a difference of optical path difference xcex94OPD at inner side end and outer side end of each of the at least one of the plurality of wide-width annular zones may satisfy a condition:
0.75xcex less than xcex94OPD less than 1.25xcex,
wherein, xcex represents a wavelength of the beam for the disc having the low data recording density.